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Re: Modified footing design

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I am not sure I understand exactly what you are describing, but here is my
initial impression.  Caisson, in my vocabulary, is a drilled in place pier.
If the existing tower is supported by the caisson, it sounds as if you are
simply expanding the support slab for an enlarged tower.  As far as
foundation design is concerned, the caisson itself needs to be checked for
the additional load, bearing and overturning, based on the allowable values
for the caisson (bearing pressure at the base of the caisson in competent
bearing material)  The "collar footing" as you describe it sounds simply
like an expanded support slab for the enlarged tower, not a footing at all.
This would be designed strictly according to concrete design for transfer of
the forces to the supporting caisson, neglecting soil interaction below the
expanded slab.

If you are trying to establish that the expanded slab participates in
sharing the soil bearing with the caisson, what are the soil parameters at
the two levels?  Are the soils even the same material? It is probable that
the two elevations will not share the load equally.  To properly mobilize
the bearing resistance at the expanded collar footing may require more
movement than the caisson will permit.

If the existing caisson is adequate for the additional load, I would design
the expanded support independent of the soil as described above.  If the
caisson is not adequate, adequate soils information is required to produce a
reasonable basis of design.


Paul Feather

----- Original Message -----
From: "Phillip Rogers" <phillip622(--nospam--at)yahoo.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Saturday, December 15, 2001 6:52 PM
Subject: Modified footing design


> I am a young engineer interested in learning more
> about foundation engineering.  I  understand spread
> footing design.  My problem is finding the method of
> determining the adequacy of a modified footing.
> Specifically:
>
> There is an existing 4'X 13.5' caisson supporting a
> tower structure.
> The problem lies in the need to modify this footing to
> make up for the tower's extension and additional load.
> The proposed method is to place an 8' square, 2' thick
> collar footing around the caisson.  This addition is
> held on to the caisson by steel bars drilled into
> place.  This addition is 1.5' underground and 3.5' to
> the base.  The addition is reinforced with #11 bars
> top and bottom (3 on the outer part of all sides of
> the caisson and 6 U-Bars in the middle sections on all
> sides).
>
> How does one go about checking the reinforcment and
> dimensional adequacy for such a modification?  What
> kind of calculations must be done that differ from
> that of a spread footing design?
>
> I understand the need for such a design.  But where
> can I find the supporting calculation methodology for
> such a case?
>
> thanks
>
>
>
>
>
>
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